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Appendix: 21-cm Line Interferometry and the Dark Matter Problem

[Reprinted from the Magellanic Times, Day 7, 22 July 2003 - the newspaper edited during the 25th General Assembly of the IAU in Sydney], minor editing afterwards in [].

"Write me a piece, Albert. I'd really appreciate it, I won't have time to do it myself." I am sitting opposite Seth Shostak, editor of the Magellanic Times, whom I have known for a long time, and known of for even longer. He and I owe a lot of intellectual indebtness and gratitude to a quiet man who patiently taught us the principles of 21-cm HI interferometry, David Rogstad. Rogstad was one of the first to use a two element radio aperture synthesis interferometer to study the 21-cm line emission of nearby galaxies (he was LOC co-chair John Whiteoak's first student.)

Seth did his thesis on detailed synthesis observations of 5 late type galaxies with the Owens Valley Radio Observatory (OVRO) twin element interferometer. Dave Rogstad moved for a few years to the Kapteyn Laboratorium in Groningen in the late sixties, where he guided a couple of students in various projects - Henk Olthof on warps, Arnold Rots on data of NGC 6946 and IC 342, and me. He himself worked on his OVRO data of M101.

For me, it was my first research project. I had to read reams of charts to mark off phase zero measurements of fringes from the just completed Westerbork telescope. Rogstad taught me computing and offered valuable advice ("beware, if you start computing, you stop thinking.") After this, he gave me his 21-cm line data of M82 to reduce, along with all his programs - on punched cards. I was enthusiastic about this, looked up the literature on this "exploding galaxy", and learned about the galaxy rotation work of Burbidge, Burbidge and Prendergast. The M82 HI data were interesting, and Dave encouraged me to speak about them at a YERAC (Young European Radio Astronomy Conference) in Dwingeloo.

Dave's work on M101 stimulated the young staff members in Groningen, led by Ron Allen, to push for a temporary receiver to do 21-cm line work on nearby galaxies with the Westerbork telescope - which was not explicitly constructed for this purpose. By the time this project was completed, I was fortunate enough to use it for my thesis work on a pilot HI study of spiral galaxies of various morphological types. That was more or less [considered] a backwater project, the main attention for 21-cm line work being the study of spiral structure of M51, M81 and M101 ("testing the density wave theory.")

Even so, I witnessed the debate about rotation curves in the outskirts of galaxies using HI data. I vividly remember a seminar by Mort Roberts, arguing that M31's rotation curve was flat, and all the young Turks of the Kapteyn lab arguing that he picked up signal of the main disk through the sidelobes of his single dish antenna (Ed Salpeter was subjected to a similar treatment a few years later [at the IAU Symposium 77 in Bad Münstereiffel] when he reported on his Arecibo data, which had some after-effects if his Annual Reviews autobiography is any guide). So 21-cm line interferometry was the creed.

My thesis work, with its basic result that the rotation curves of spiral galaxies of all morphological types stayed flat (or were even still rising) beyond the optical image, helped settle the debate of the presence of dark matter in the outer parts of spirals. I also was fortunate to be able to go to Palomar with Piet van der Kruit, to get Halpha rotation data in the inner parts, and to obtain plates for surface photometry to determine the luminosity profile and the mass-to-light ratio as function of radius.

But the questions remain - how was the dark matter debate conducted, who said what when and why, and who had the "correct" (i.e. present day accepted) picture first? This is not so easy as some people would have it. In Historical Development of Modern Cosmology (ASP conference series Vol. CS-252), I read two articles - one by Sidney van den Bergh and one by Jaan Einasto - and there seems to be little overlap. So there could be different stories, and different people were convinced at different times of the presence of the dark matter in spirals - just like the various events and characters which made up the French Revolution. In any case, as far as the HI line rotation curves is concerned, I can only cite Vera Rubin et al.'s (1978 paper where she writes that "Mort Roberts and his collaborators deserve credit for first calling attention to flat rotation curves", credit he got only sparingly. What is clear now is that there is little need for a dark halo to fit the optical rotation curve data, a conclusion [again] reached in an ApJ paper as recently as 2000. Mort Roberts himself confided to me once that he thought that my thesis work vindicated his contention that rotation curves of giant spirals are flat. Ostriker and Peebles, who pushed the idea for dark halos, first in a 1973 paper on disk stability (a paper which nowadays cannot withstand close scrutiny), and then, with Yahil, in their classic 1974 paper on various indicators pointing to a linear increase of the mass of a giant galaxy with radius, cite for the rotation data the papers by Roberts and Rots (1973) and Rogstad and Shostak (1972).

Which brings me back to Seth Shostak. Contrary to my thesis, which I had printed in 500 copies, and with its handy format became quite in demand (my thesis advisor had his copy stolen from his office somewhere in the 1980s, he told me recently), Shostak's thesis (1971) is not widely available. I found a copy of it in the VLA library, so I started reading it carefully, and yes, it contained the flat rotation curve of NGC 2403, and its implication that the volume density of matter drops off as r-2 (mass rises linearly with radius, therefore). So he had it! Okay, only for one case, but even so; and it was a clean observation with an interferometer, so unlike Roberts' observation of M31 (the primary piece of evidence in the Roberts and Rots (1973) paper). Oh well, it is unclear to me sometimes how scientific evidence is accepted in certain circles. Sometimes one has a result, correct after all, but at a time too far ahead of the pack, which is not yet ready to absorb it.

In any case, the amusing part of Seth's thesis is in the very end. A brief acknowledgement to Gordon Stanley, then director of OVRO. A warm thanks to David Rogstad for his patient advice. And then this fantastic flight of fancy - "this thesis is dedicated to NGC 2403 and its inhabitants, to whom copies can be furnished at cost". No wonder this man is now working on SETI! He just wants to know why nobody came along thus far to claim a copy. If somebody from that galaxy had come along, Seth would have been the most famous man on earth, offered an autographed copy of his thesis for free, and arranged for a public lecture at the current General Assembly. John Whiteoak would have been happy to schedule that, and the Harbourside Auditorium would have been way too small!

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